1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assemblies and more particularly to an improved loading of spacer grid springs for spacing and cradling fuel rods in a nuclear fuel assembly.
2. Description of the Prior Art
In a nuclear fuel assembly, spacer grid assemblies are used to precisely maintain the spacing between the fuel rods in a nuclear reactor core, to prevent rod vibration, and to provide lateral support for the fuel rods. Conventional spacer grid assembly designs include a multiplicity of metal grid straps, interlocked into an egg-crate configuration designed to form cells through which fuel rods (standard cells) and control rod guide thimbles (thimble cells) pass. Slots are often utilized to effect the interlocking engagement between grid straps. Each standard cell provides support for one fuel rod at a given axial location through the use of relatively resilient grid springs and relatively rigid protuberances (dimples) formed in or attached to the metal grid straps. A peripheral strap may be used to enclose the interlocked grid straps in order to impart strength and rigidity to the fuel assembly and to provide grid spring and dimple locations for the peripheral cells. In order to minimize the lateral displacement of fuel rods during ooperation and to improve the fuel characteristics of a fuel assembly, a number of such spacer grid assemblies may be spaced along the fuel assembly length. These grids are generally held in place by attachment to the control rod guide thimbles by known techniques.
Examples of such spacer grid assemblies may be found in U.S. Pat. No. 3,389,056 to E. Frisch, U.S. Pat. No. 3,713,971 to Van Santen et al, U.S. Pat. No. 3,944,467 to Biermann et al, U.S. Pat. No. 4,224,107 to Delafosse et al, or U.S. Pat. No. 4,474,730 to Hellman et al.
During operation of a nuclear reactor, the grid springs and dimples undergo exposure to radiation. Depending on the amount of irradiation, the material used for the grid springs will lose some of its initial spring force, thus permitting the fuel rods to vibrate and chatter against the springs and dimples. If vibration is severe, the result may be fretting of the fuel rod cladding.
Some known grid assemblies are constructed of zirconium or zirconium alloy. (Both terms hereinafter generally are referred to interchangeably as Zircaloy.) Some grids are fabricated from Inconel or Zircaloy/Inconel composites. Although Inconel is a preferred material for the springs because it has a high spring force which is relatively insensitive to radiation-induced relaxation, it has a high neutron absorption cross-section. Thus, the use of Inconel in large quantities results in a significant economic penalty. Zircaloy, on the other hand, has a low neutron cross-section but relaxes under high heat and radiation and thereby initially loses almost 30% of its spring strength due to high heat (e.g., 600.degree. F.), and typically loses up to nearly 90% of its remaining spring force after one year of intense radiation.
In a typical reactor having an upper and lower grid and a number of intermediate grids, Inconel is used in the lower grid because, even though it has a high neutron absorption cross-section, a strong, long-term spring force is necessary to prevent vibration-induced fretting of the fuel rods due to the high turbulence at the inlet of the fuel bundle. Additionally, Inconel is used in the upper grid near the top of the fuel rod where there is relatively little radiation in the gas plenum area. In both cases, Inconel retains its resisting strong spring force under irradiation. Zircaloy is often used in intermediate grids where strong spring force is not as critical as at the ends, to space and support the fuel rods without significant neutron absorption.
Although mixed arrangements of Inconel and Zircaloy grids perform well, one significant problem occurs, namely high rod bow, caused in part by a bending moment produced by the offset lateral application of relatively strong upper and lower grid spring and dimple forces at the ends of the rods. Rod bow is aggravated by axial compression of the rod. As a fuel rod undergoes irradiation, it tends to grow axially. As a rod grows, the relatively strong spring force of the upper grid tends to resist the growth and thereby places the rod in compression. The rod bows as a result of the axial compressive force and the bending moment, coupled with radiation-induced relaxation of the intermediate Zircaloy grid springs, which results in insufficient intermediate spring force to keep the rods straight at the intermediate grid locations.
High bowing is undesirable because it results in significant departure from nuclear boiling (DNB) heat transfer penalty, accelerated Zircaloy corrosion if two fuel rods touch, and a negative customer perception if the fuel rods are not visibly straight.